Radio communications systems of today typically employ a modulation scheme, in which an intelligence-bearing signal is superimposed or mixed into a propagating carrier signal.
For some communications systems, including a GSM (Global System for Mobile Communications) or GPRS (General Packet Radio Service) system, the sole choice of available modulation scheme has been GMSK (Gaussian Minimum Shift Keying). GMSK is a kind of constant-envelope phase modulation, where transmitting a zero bit or one bit is represented by a change in the phase. Thus, every transmitted symbol represents one bit.
Introduction of the EDGE (Enhanced Data rates for GSM Evolution) technology into a GPRS systems provides another modulation scheme to be employable for radio communications, namely 8-PSK (8-state Phase Shift Keying). 8-PSK enables reuse of the channel structure, channel width and the existing mechanisms and functionality of the GMSK-using GPRS system. However, 8-PSK enables higher bit rates per time slot than those available for GMSK. 8-PSK is a linear method that uses phase and amplitude modulation, in which three consecutive bits are mapped onto one symbol. Although the symbol rate remains the same as for GMSK, each symbol now represents three bits instead of one, thus, increasing the total data rate by a factor of three.
An EGPRS (Enhanced GPRS) system having access to both GMSK and 8-PSK modulation can use nine different modulation coding schemes, MCS1 to MCS9. The lower four coding schemes use GMSK whereas the upper five use 8-PSK. These nine MCS use different error correction and, consequently, are adapted for usage under different radio environment conditions. Generally, in good radio environments a more aggressive (less error correction, 8-PSK-associated MCS) coding scheme can be used to provide a higher user data rate, whereas with a poor radio link environment a coding scheme with more error correction (GMSK-associated MCS) and lower user data rate is typically used.
The EGPRS system also employs link quality control functionality denoted link adaptation. Link adaptation uses radio link quality measurements from a mobile unit to select the most appropriate modulation coding scheme for transmission of subsequent data packets to the mobile unit. Such a measurement report from the mobile unit includes only link quality measurements, e.g. BEP (Bit Error Probability), for the modulation that has been used since a last measurement report. However, since the link quality measurements are dependent on the particular modulation scheme employed, the network has to make an assumption about the relative performance of GMSK modulation and 8-PSK modulation. For example, if the network receives a report with a BEP for data received by the mobile unit and modulated by GMSK, the network “maps” this GMSK BEP to a corresponding estimated 8-PSK BEP value.
A major problem with this prior art procedure is that the network uses a single fixed modulation scheme BEP mapping. However, the actual relative performance of GMSK and 8-PSK modulation may vary from one radio environment to another. This problem also aggravates in situations where the mobile unit employs interference-suppressing techniques for data modulated using one of modulation schemes but not for the other scheme(s). A typical example is the so-called SAIC (Single Antenna Interference Cancellation) or SAIR (Single Antenna Interference Rejection) technique that presently only can be used in conjunction with GMSK modulation. Since SAIC can improve the GMSK performance up to about e.g. 9 dB depending on the actual radio environment and traffic load, a relative uncertainty of up to 9 dB is introduced for the GSMK BEP mapping into a 8-PSK BEP value. As a consequence, the network may select a non-optimal modulation coding scheme for a mobile unit, possibly resulting in a loss of radio blocks intended to the mobile unit.